Refrigeration Systems Operation and Maintenance PDF

4th Class Edition 3 Part B Unit 9 Chapter 4 Refrigeration Systems Operation and Maintenance Learning Outcome When you complete this chapter you should b...

4th Class Edition 3 Part B Unit 9 Chapter 4 Refrigeration Systems Operation and Maintenance Learning Outcome When you complete this chapter you should be able to: Describe the operating principles and maintenance of refrigeration system. Learning Objectives Here is what you should be able to do when you complete each objective: 1. Discuss refrigeration auxiliaries. 2. Describe refrigeration system leak test procedures. 3. Describe how a refrigeration system is dried and charged prior to start-up. 4. List the steps for adding oil to an in-service refrigeration compressor. 5. Describe the start-up and shut-down procedure for a compression refrigeration system. 6. Describe operational log sheets and preventative maintenance procedures for refrigeration systems. 7. Describe how a refrigeration system is purged of noncondensable gases. 8. Discuss refrigeration condenser operation and maintenance requirements. 9. Explain typical problems and resolutions related to refrigeration systems. 4-1  Refrigeration Systems Operation and Maintenance Chapter 4 Chapter Introduction It is important for Power Engineers to know the layout and types of equipment unique to the refrigeration systems they operate. The location of valves, separators, economizers, and oil pots, as well as other systems components, must be learned and committed to memory. The function of these components must be well-understood, so that the equipment can be started, stopped, operated, and maintained in a safe and economical fashion. Power Engineers must also continuously familiarize and re-familiarize themselves with the various codes and standards that apply to their profession. Codes such as CSA B52 and ASME B31.5 apply directly to the installation, repair, and operation of refrigeration systems. IIAR standards are particularly helpful with regard to ammonia system installation, repair, commissioning, operation, and maintenance. These are as important to the refrigeration plant operator as the ASME BPVC is to the steam plant operator, and should be part of the refrigeration plant engineer’s professional library. This chapter contains generic operation, maintenance, and troubleshooting guidelines. These must not be regarded as universal practices. There is no better source of system specific information as the data provided by equipment manufacturers. Manufacturer and supplier websites are invaluable sources for information about operating, maintaining, and troubleshooting specific refrigeration equipment. Always follow site specific procedures. 4-3 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 1 Discuss refrigeration auxiliaries. Pressure Gauges Pressure gauges indicate the conditions inside a refrigeration system. They not only show the evaporator and condenser pressures, they (under normal conditions) indicate the corresponding saturation temperatures. The CSA B52 Mechanical Refrigeration Code does not require permanently installed pressure gauges on any refrigeration system. However, in ammonia refrigeration systems (and larger HFC and HCFC systems), permanent gauges are normally installed. One gauge is connected to the high pressure side of the system, and another to the low-pressure side. These gauges are usually connected to the compressor discharge and suction, and are mounted on or near the compressor. They should be equipped with pulsation dampeners to prevent flickering of the pointers due to pressure pulsations. These pulsations lead to premature wear of the gauge internal mechanisms. Pressure gauges are rarely installed on low capacity commercial refrigeration systems. However, these systems usually have provision for attaching gauges. Double-seated compressor service valves have back-seats and flared ends to permit the installation of service gauge sets. When these connections are not available, a permanent connection may be added to permit service gauge installation. These connections consist of a saddle, a piercing valve, and a service gauge connector. The saddle can be bolted or brazed onto the refrigerant pipe. After securing the valve in place, the piercing valve is turned inward to pierce the refrigerant tube. Then, the gauge set can be attached, and the valve opened to measure the refrigerant pressure. A typical gauge connection is shown in Figure 1. Three gauges, indicating the compressor suction, compressor discharge, and oil pump discharge pressure are mounted on a separate gauge board. This type of connection reduces the effects of compressor vibration. Figure 1 – Gauge Connections Suction Compressor Oil Discharge Pressure Discharge Pressure Suction Pressure The oil pressure gauge is connected to the oil pump discharge. Oil pressure differential, which is the useful oil pressure, can be found by subtracting the compressor suction pressure from the oil pressure on the gauge. 4-4 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Accumulator/ Surge Tank / Surge Drum On some refrigeration systems, the metering or control action is too slow to keep pace with load changes. Also, capillary tubes cannot shut off the refrigerant flow when evaporator load is light or when the compressor cycles off. In both cases, liquid may occasionally enter the suction line and damage the compressor. An accumulator is a simple liquid trap located in the compressor suction line. It collects and holds the liquid so that it does not enter the compressor. Accumulators must not be insulated. This is so heat may be absorbed through the accumulator wall. The heat vapourizes the trapped liquid and returns it to the compressor suction. If insulated, liquid refrigerant could accumulate and eventually become entrained in the compressor suction. An accumulator is shown in Figure 2. Figure 2 – Accumulator Installation Discharge Line Suction Line to Condenser from Evaporator Small Bleed Hole Compressor Accumulator for Oil Return Oil Separator During operation, a certain amount of oil leaves the compressor along with the high-pressure vapour. It is therefore necessary to separate the oil from the vapour and return it to the compressor crankcase. This maintains the oil level in the crankcase and reduces the need for oil addition. 4-5 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Oil will enter the condenser if it is not separated from the vapour. Here, the oil coats the heat transfer surfaces, impeding heat transfer. This will result in high compressor discharge pressure and increased compressor power consumption. In order to avoid this problem, an oil separator should be installed in the discharge line between the compressor and the condenser. One design of oil separator is shown in Figure 3. Figure 3 – “Sterne” Oil Separator Vapour Vapour Outlet Inlet Oil Return to Recovery Drum or Compressor Stop Crankcase Valve Stop Valve Sludge Accumulated Drain Lubricant and Sludge On entering the separator casing, the refrigerant vapour is given an abrupt change in flow direction. As a result, entrained oil droplets are thrown out of the vapour stream, collected at the bottom of the separator and returned to the compressor crankcase. The sludge drain is used for blowing off sludge or scale that collects in the bottom of the vessel. At regular intervals, the oil drain valve should be cracked open to allow the oil in the separator to return to the crankcase. A sight glass allows the operator to determine the oil level in the accumulator. 4-6 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Figure 4 shows an oil separator used in an ammonia refrigeration system. The hot vapour discharged from the compressor enters the separator at the right-hand side of the vessel. The accumulated oil is returned to the compressor with the piping arrangement on the bottom left-hand side of the vessel. The return piping arrangement has an isolation valve, a dirt leg, and a filter. The filter catches impurities to prevent them from entering the compressor sump. Figure 4 – Oil Separator in Ammonia Refrigeration System Refrigerating systems using oil-miscible refrigerants, such as HFCs and HCFCs, also use oil-separating devices in the compressor discharge line. One type is shown in Figure 5. It is only possible to extract oil from these refrigerants while they are in the vapour state. An automatic float trap may be installed to reduce the need for an operator to manually drain the oil. When sufficient oil collects, the float opens the drain valve permitting the oil to return to the compressor crankcase. To prevent HFCs and HCFCs from condensing and draining to the crankcase with the oil, the trap must be kept hotter than the condensing temperature. This is done by placing the separator close to the compressor, and protecting it from cold air drafts. 4-7 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 5 – Oil Trap for HFC and HCFC Systems Vapour Inlet Vapour Outlet Gauze Baffle Screen Float Valve Oil Drain Suction Strainer Particles of foreign matter, such as line scale or corrosion particles, may be carried by the refrigerant vapour into the compressor. This may prevent proper seating of compressor valves or cause damage to compressor parts. Strainers are installed in compressor suction lines to keep these particles from entering compressors. Figure 6 shows a cross-sectional view of a typical strainer. It has a fine mesh screen basket placed in a strainer housing. It can be removed for cleaning without disconnecting any piping. Many smaller compressors, especially those of the hermetic type, are equipped with built-in suction strainers. Figure 6 – Suction Strainer 4-8 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Filter-Drier The main function of a filter-drier is to remove moisture from liquid refrigerant. If not removed, moisture in a refrigeration system causes the following problems: ice buildup in the metering device, disrupting its operation acid formation, resulting in »» corrosion »» sludge formation in the compressor crankcase »» deterioration of motor insulation in hermetic and semi-hermetic compressors. Filter-driers also remove particulate and scale from the liquid refrigerant before it flows to the metering device. This protects the metering device from plugging. Figure 7 shows a cross-sectional view of a filter-drier used in a small refrigerating system. It has a sealed shell containing a drying agent (a desiccant), which removes moisture either by adsorption (silica-gel or activated alumina) or by chemical reaction (calcium sulphate). The filter-drier is installed in the liquid refrigerant line ahead of the expansion valve. This filter-drier must be replaced when the desiccant reaches its moisture holding capacity. Driers in larger systems can be opened up to replace the desiccant, which is supplied as a cartridge. Figure 7 – Refrigerant Drier Desiccant Screen Shell Sight Glass and Moisture Indicators It is common practice to install a sight glass in the liquid refrigerant line of a commercial refrigerating system to observe the liquid flow. The sight glass is a small housing equipped with one or two lenses. When the system contains insufficient refrigerant, the pressure drop through the liquid line to the metering device increases. This causes vapour to develop in the liquid line. This vapour shows up in the form of bubbles as it passes through the sight glass. Vapour bubbles also show if the flow in the liquid line is restricted, causing a pressure drop and resulting in part of the liquid flashing into vapour prematurely. 4-9 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Moisture in liquid refrigerants is detected by the use of special chemicals which change colour in the presence of moisture. The moisture indicator is usually combined with the sight glass. The indicator is simply of a chemical dot placed under the sight glass lens, so it is exposed to the liquid. The chemical dot changes colour when moisture is present. The sight glass dot shown in Figure 8 changes from green to yellow when moisture is present. This indicates that the filter-drier needs replacement. Figure 8 – Sight Glass with Moisture Indicator The location of the sight glass depends on the purpose for which it is used. If it is used to indicate whether the system is fully charged, the sight glass will be installed at the receiver or liquid receiver outlet. When it is used to indicate if the refrigerant contains any flash vapour, the glass is installed before the expansion valve. If the glass is combined with a moisture indicator, and the liquid line from the receiver has a drier, the glass should be installed between the drier and the expansion valve to allow the operator to see if the drier is plugging or desiccant expiration has occurred. Economizers An economizer is a heat exchanger used to transfer heat from the relatively warm liquid flowing to the evaporator to the vapour being drawn from the evaporator. Economizers are used for three reasons: 1. They reduce the temperature of the liquid refrigerant. This reduces the amount of flash gas generated in the evaporator. This increases the evaporator capacity (net refrigerating effect). 2. They prevent flashing of the liquid due to the pressure drop that occurs as the refrigerant flows through the liquid line. 3. They increase the temperature of the vapour passing though the economizer. Although the flow of liquid into the evaporator is automatically controlled to match the load, some liquid may be carried from the evaporator into the suction line when rapid load fluctuations occur. If any liquid passes through the evaporator, the economizer will evaporate it before it reaches the compressor suction. In this way, the economizer can help prevent liquid slugging of the compressor. 4-10 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Figure 9(a) shows a refrigerant heat exchanger used in small and medium capacity refrigeration systems, while Figure 9(b) shows the location of the heat exchanger. Figure 9 – Economizer Suction Line Vapour from to Compresser Evaporator Liquid Refrigerant Liquid Refrigerant from Condenser Expansion Valve (a) Heat Expansion Valve Exchanger EVAPORATOR Liquid Flow Suction Line Thermal to Compressor Bulb (b) To eliminate the use of a separate heat exchanger in low capacity systems equipped with a capillary tube for liquid refrigerant control, it is common practice to solder the capillary tube to the suction line so heat transfer can take place. This is shown in Figure 10. Figure 10 – Soldered Liquid and Suction Lines Evaporator Suction (Gas) Line Strainer Capillary Tube Liquid Two Lines Soldered Liquid from Condenser Gas from Evaporator 4-11 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Distributor When a large direct-expansion type evaporator has more than one refrigerant circuit, the liquid refrigerant entering the evaporator must be evenly distributed to each one. This will ensure cooling occurs equally at all points in the evaporator coils. A distributor, placed in the liquid line directly downstream from the expansion valve, ensures each coil is fed refrigerant equally. A cutaway view of a pressure drop type distributor is shown in Figure 11. Figure 12 shows both horizontal and vertical views of a manifold type distributor connected to an evaporator. Figure 11 – Liquid Refrigerant Distributor Retainer Nozzle Body Ring Exploded View Tubing Figure 12 – Distributing Manifold Manifold Expansion Valve Distributor Liquid from Condenser 4-12 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Vibration Absorber Vibration and noise are created by refrigeration system components, such as the compressor, forced convection evaporator, or condenser. The vibration and noise are transmitted through rigidly connected piping. Sometimes, the noise is amplified to objectionable levels. By connecting the piping to the main components of the refrigeration system using vibration absorbers, transmission of vibration and noise is prevented. A typical vibration absorber is shown in Figure 13. Figure 14 shows where they are commonly installed. The vibration absorbers should be located as close as possible to the compressor for maximum effect. Figure 13 – Vibration Absorbers Figure 14 – Vibration Absorber Location Vibration Absorbers Suction Discharge Semi-Hermetic Compressor Refrigerant Piping And Tubing In general, the type of piping and tubing material used in a refrigeration system depends upon the size of the piping and tubing required, the refrigerant used, and the application. The CSA Standard B52, Mechanical Refrigeration Code, requires that piping, tubing, valves, fittings, and related parts should conform to the minimum requirements set forth in the ASME Code B31.5: Refrigeration Piping and Heat Transfer Components. Ammonia piping has several specific requirements, covered under the International Institute of Ammonia Refrigeration (IIAR) standard “IIAR 2 Standard for Safe Design of Closed-Circuit Ammonia Refrigeration Systems.” At times, these codes and standards disagree. Best practice dictates that the most restrictive code should be referenced, in order to maintain the highest safety design standards. From a jurisdictional standpoint, however, Canadian code always takes precedence unless stipulated otherwise in legislation. 4-13 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Materials Proper material must be used for piping. ASME B31.5 lists a variety of ferrous and non-ferrous materials that may be used for refrigeration system piping. All materials must be suitable for low temperature service. In other words, materials that become brittle at low temperature cannot be used. As well, materials must be compatible with the refrigerant. Ammonia Service Piping or tubing must be steel, and listed as permissible in ASME B31.5. The CSA B52 Code, as A E well as the ASME B31.5 Code and the IIAR Standards, prohibits copper and copper alloy pipe and tube. Steel pipe should be Type S (seamless) or Type E (electric resistance welded). The ASME B31.5 code prohibits Type F (furnace butt-weld) pipe in ammonia service, except for water-based secondary coolants (brine). Galvanized pipe should not be used. IIAR 2 stipulates that zinc should not be used to contain, nor should it contact, ammonia. Piping sizes DN 150 (NPS 6) and smaller must be at least Schedule 40, except for pipe smaller than DN 50 (NPS 2), which must be at least Schedule 80. If the pipe is to be joined with threaded connections, Schedule 80 must be used as a minimum, regardless of the pipe diameter. The ASME B31.5 code prohibits malleable iron and cast iron fittings in ammonia service. IIAR Standard 2 states that only Class 3000 or stronger fittings shall be used when attachments are made by socket welding or threading. Tubing, if used, can be carbon steel or stainless steel. IIAR Standard 2 limits the use of tubing to compressors, compressor packages, and packaged systems. HFC and HCFC Service All HFC and HCFC refrigerants may use either copper or steel piping. Steel should be used for all larger diameter piping due to its higher strength. CSA B52 permits only Type K or L hard-drawn copper tubing when the installed piping is exposed to mechanical injury. Soft annealed copper tubing can also be used; however, it must not A E exceed 35 mm O.D. (1-3/8 inch). ASME B31.5 permits gray iron, malleable iron, and ductile iron fittings in HFC and HCFC refrigeration systems; however, they are limited to service above -20 °C. Ductile iron fittings cannot be used above 6895 kPa. Piping Connections In general, the least number of fittings should be used. This reduces piping system pressure drop and the possibility of leaks. ASME B31.5 permits the following ways of connecting refrigeration system piping: Threaded connections Welded connections Flanged connections Brazed connections Flared connections Soldered connections Compression fittings Flareless connections 4-14 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Threaded joints should be made with tapered pipe threads. When tightening a threaded connection, the joint must never be loosened for the purpose of alignment. Threaded joints should not be used for pipe sizes over DN 100 (NPS 4). Threaded fittings can be seal welded after installation. Threaded copper and brass fittings can be used, but not in ammonia service. After making up the threaded joint, all exposed threads must be coated with grease or other suitable coating to protect the exposed threads from corrosion. Welding is the most commonly used method of joining steel piping. Welded joints provide leak-free connections. Fittings can be socket welded or butt welded. Threaded fittings can be seal-welded, as long as the joint is assembled without any type of thread sealant, lubricant, or dope. Welding must only be performed by certified and qualified pressure welders. The class of flange selected for flanged connections must be suitable for the design pressure. Connections must be made up carefully to ensure the gasket loading is equal. Gaskets must be compatible with the refrigerant chemistry and system pressure. Though ASME permits soldering, CSA B52 prohibits all soldered joints in copper piping and tubing. Brazing metals are stronger and better suited for higher temperature operation. Lengths A E of tube are swaged at one end and then brazed together. Copper tubing is often silver brazed. Flare fittings and compression fittings are commonly used for pressure sensing lines, oil return lines, and other applications where small diameter tubing is sufficient, and occasional disassembly is required. Fittings and tubing may be steel or copper. Ammonia Service According to ASME B31.5, threaded joints shall not be used for piping larger than DN 50 (2 in). Unions must be forged steel, without a brass seat. Flanges, when used, must be the raised face type. Gasket material must be compatible with ammonia, and suitable for the pressure application. Joint compound must not contain copper or copper-alloys. Other Piping System Considerations (Non-Code) Piping should be kept clean to minimize corrosion. Repaint bare piping on a regular basis to help prevent corrosion. Insulation should be repaired or replaced as necessary. Horizontal lines should slope downward in the direction of refrigerant flow. The minimum recommended slope should be 4.2 mm per metre. As good practice, flared compression fittings may be used for joining soft-temper copper tubing up to 19 mm outside diameter (O.D.). Above this size, and for hard temper copper tubing, joints should be made with silver brazed fittings. Thin-wall steel tubing may be joined by the use of either flared or compression fittings. Stop Valves Manual stop valves are used in refrigeration systems to isolate parts of the system, or transfer refrigerant from one part of the system to another. This is so that maintenance and repair work can be performed without releasing refrigerant to the environment. CSA B52 Code states that Systems containing more than 50 kg of refrigerant shall have stop valves installed at the following A E locations: on each suction inlet of each compressor, compressor unit, liquid refrigerant pump, or condensing unit; on each discharge outlet of each compressor, compressor unit, liquid refrigerant pump, or condensing unit; on each inlet of each liquid receiver, except for self-contained systems or when the receiver is an integral part of the condenser or condensing unit; on each outlet of each liquid receiver; and on each inlet and outlet of condensers when more than one condenser is used in parallel in the system. 4-15 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration These valves are special for refrigeration service. They are designed to prevent the escape of refrigerant to the atmosphere, yet remain easy to operate. The diaphragm in the valve shown in Figure 15 prevents the escape of refrigerant along the valve stem to atmosphere. This valve also has a back seat to prevent the escape of refrigerant along the valve stem into the space between the upper seat and diaphragm when the valve is fully open. Valves with packed stems (Figure 16) are constructed with a back seat. When fully open, the back seat is closed, preventing refrigerant leakage. Some valves also have a scraper to prevent dirt and ice from damaging the packing gland. Figure 15 – Diaphragm Type Refrigeration Valve Diaphragm Back Seat As an added precaution against leakage along the valve stem, many packed valves are also equipped with a valve cap that covers and seals the valve stem. It must be removed before the valve can be operated. The valve shown in Figure 16 has a combination valve cap and valve stem wrench. To operate the valve, the stem cap is removed and turned upside down. The square recess at the top of the stem cap fits on the square end of the valve stem. The stem cap acts as a handle to open and shut the valve. After the valve is operated, the stem cap is turned over and replaced to prevent refrigerant leakage. Figure 16 – Refrigeration Valve with Stem Packing Stem Cap Valve Stem Back Seat 4-16 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Receiver Gauge Glass Larger liquid receivers should be fitted with gauge glasses so the liquid level can be readily seen. The CSA B52 Code states that liquid level gauge glasses must have automatically closing shut-off valves to prevent loss of refrigerant if the glass breaks. This type of valve is shown in Figure 17. A E Figure 17 – Safety-Type Gauge Glass Fitting Gauge Glass Lower Gauge Heavy Nipple Valve Steel Ball Ball Seat Unseating Spindle The gauge glass valves work the same way as those used for steam boilers. They are installed on both the vapour space and the liquid space connections. If the glass breaks, the pressure of the refrigerant forces the steel ball to press tightly against the ball seat, thus preventing the loss of refrigerant. After the glass is replaced, the balls are unseated with the unseating spindle, allowing the liquid to flow back into the gauge glass. The unseating spindles are back seated when fully open, to prevent leakage along the spindle and packing during normal operation. CSA B52 also requires gauge glasses to be adequately protected against damage. This will reduce A E the likelihood of having to deal with a broken gauge glass. Because most refrigerants are clear in colour, reflex glass (Figure 18) is used to indicate level. Reflex glass uses prismatic action to indicate the presence of clear liquids as black. This makes it easy to see the level. Figure 18 – Reflex Glass Level Indicator 4-17 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Another common method of indicating liquid level is to use multiple “bull’s-eye” sight glasses (Figure 19). The glass is either a reflex design or uses small brightly coloured flotation balls to indicate the refrigerant level. In cases where one glass has a ball at the top of the glass and the glass above has a ball at the bottom, the level is between the two glasses. Figure 19 – Bull’s-Eye Sight Glasses Liquid Level in Between 0.31 Sight Glasses Receiver Level Indicators Purge/Charging Valve The purge valve and charging valve are usually packed or diaphragm type angle valves. The purge valve is used to vent non-condensable gases from the system. The charging valve is used to charge the system with refrigerant. The open ends of these valves are usually capped to prevent escape of refrigerant from the system when the valves are not in use. Pressure Relief Devices Since a refrigeration system, regardless of its size, is a closed pressure system, the possibility always exists that the pressure in the system or in its components may build up excessively due to either extreme temperature conditions, malfunctioning controls or inadvertently closed stop valves. Pressure buildup above the design pressure could result in rupture of some part of the system. General Provisions To prevent over-pressurization, the CSA B52 Mechanical Refrigeration Code Part 7.3 requires A E every refrigeration system to be protected by one or more pressure-relief devices. In particular, those parts that contain liquid refrigerant, are larger than 152 mm (6 in) internal diameter, and can be isolated using valves, must be protected from over pressurization. These pressure-relief devices must be connected as directly as possible to the part or parts of the system being protected. 4-18 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 All pressure-relief devices in refrigeration service must discharge to the outside of the building, if the system contains: a Group A3 or B3 refrigerant; A E more than 3 kg (6.6 lb) of a Group A2, B1, or B2 refrigerant (such as ammonia); or more than 50 kg (110 lb) of a Group A1 refrigerant (such as R-134a). No stop valve is permitted between a relief device and the part of a system it protects. Two of the most common pressure relief devices used in refrigeration systems are the fusible plug and the spring-loaded relief valve. Fusible Plugs Fusible plugs are commonly used in smaller systems, and are only used to protect smaller volume components (containing less than 0.085 m3 of refrigerant). The fusible plug is a device containing an alloy that melts at a specified temperature to relieve pressure. When the plug melts, the entire refrigerant charge escapes. A new plug and a new refrigerant charge are required before the system can be put back into operation. Fusible plugs may be located above or below the liquid refrigerant level, except on the low side. They must be marked with the melting temperature in degrees Celsius. Fusible plugs are primarily used to protect from explosion in case of fire. They are not reliable or accurate over-pressure relief devices. Consider a low-temperature fusible plug that melts at 74°C. The minimum high side design temperature for R-134a using an air-cooled condenser is 1282 kPag. At 74°C, when the fusible plug releases, the pressure in the receiver will be around 2350 kPa, which is nearly double the minimum high side design pressure. This is well within the factor of safety for the vessel; however, due to the significant over-pressurization that may occur, the vessel would no longer be safe for continued service. Fusible plugs are non-reclosing pressure relief devices. When activated, they release large quantities of refrigerant until the system is empty, or the component they serve is isolated. This has significant environmental and economic impact. Therefore, reclosing pressure relief devices (such as spring-loaded safety valves) are preferred. Safety Valves Vessels Refrigeration safety valves are noticeably different from those used in steam or hot water service, because they do not have manual try levers or exposed springs. This is because refrigerant must be contained within the system, and manual tests are never conducted. In lieu of manual tests, CSA B52 Part 8.4 states that pressure-relief valves must be replaced or recertified at no longer than five year intervals. A E CSA B52 requires each pressure vessel that contains liquid refrigerant, has an internal gross volume exceeding 0.085 m3, and can be isolated using valves, to be protected by a pressure relief device. The device must have enough capacity to prevent the pressure in the vessel from rising more than 10% above the setting of the pressure relief device. This requirement negates the use of fusible plugs as the sole method of over-pressure protection for larger volume vessels. Vessels with an internal volume of 0.28 m3 (10 ft3) or greater require two full-capacity safety valves piped in parallel. These valves must be installed with a three-way valve. The three-way valve A E places only one of the two valves in service at a time. This allows the removal and replacement of the safety valves at regular service intervals. 4-19 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 20 shows a dual safety valve piping arrangement that meets this requirement. The valves shown are mounted on the oil separator of a packaged screw compressor. The three-way valve has a removable cap located over the valve stem, to prevent leakage. The valve stem must be operated fully in either direction, so that only one valve is in service at a time. The valve stem must not be kept in the intermediate position. Otherwise, both valves are exposed to service conditions, which means that both valves would simultaneously require replacement or recertification. Note that each valve has a service tag, which indicates its date of installation or last date of recertification. Figure 20 – Dual Safety Valve Installation In a refrigeration system, high side safety valves often discharge into the low side. This relieves excessive high side pressure, and recirculates the refrigerant so that it is not lost to atmosphere. However, CSA B52 Part 7.3 states this arrangement is only permissible provided that A E the high side pressure-relief devices are not affected by back pressure the low side of the system is equipped with pressure-relief devices the relief devices on the low side of the system have sufficient capacity to protect the pressure vessels that are relieved into the low side of the system, and to protect all pressure vessels on the low side of the system the low side pressure-relief devices are vented to outside of the building. Compressors Positive displacement compressors can develop enough pressure to damage piping, vessels, pipe fittings, and their own casings. To prevent this from occurring, every positive-displacement compressor with a discharge stop valve must be equipped with a pressure-relief valve. 4-20 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 The CSA B52 Code Part 7.2 requires pressure-relief devices on every positive displacement compressor. The pressure-relief device must be mounted between the compressor and the A E compressor discharge stop valve. It must have adequate capacity to prevent rupture of the compressor, and to prevent the pressure from increasing to more than 10% above the maximum allowable working pressure of any component located in the discharge line between the compressor and the discharge stop valve. The pressure-relief valve capacity is specified by the compressor manufacturer. The pressure-relief device is piped to discharge into the low-pressure side of the system or to the outside atmosphere. Figure 21 shows a safety valve for positive displacement compressor protection. Figure 21 – Safety Valve for Positive Displacement Compressor Protection 4-21 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 22 shows a refrigeration compressor safety valve with its discharge line piped to the suction side of the compressor. The safety valve can be seen on the upper right-hand side of the picture. The safety valve discharge pipe is painted orange, and terminates at the left hand side of the photo. Figure 22 – Compressor Safety Valve Piped to Suction Emergency Discharge The CSA B52 Code Annex B has guidelines for rapidly discharging refrigerants into the atmosphere during a fire or other emergency. Though optional, it is enforced by many jurisdictions. Annex B states: Systems designed for operation over 103 kPa (15 psig) and containing 182 kg (400 lb) or more A E of Group A1 or 91 kg (200 lb) or more of all other refrigerants shall be constructed so that, in an emergency, the refrigerant can be safely and rapidly discharged into the atmosphere. The emergency discharge system consists of a: A piping connection to the top of a liquid receiver or other vessel where liquid refrigerant is stored An emergency discharge valve, located outside of the building A diffuser, located at a high elevation, to spread the refrigerant vapour over a large area. 4-22 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 The emergency discharge line is connected directly to the top of the receiver or other vessel (the vapour space), as shown in Figure 23. There must be no other valve between the emergency valve and the vessel. An emergency switch to stop the refrigeration equipment must be installed beside the emergency valve. The switch can be seen beside the valve in Figure 24. The emergency valve shall be installed on a horizontal pipe, in a glass fronted box painted bright red, outside of the building and so placed that it cannot be operated by anyone other than the plant operator, a firefighter, or a person who could be called on to open the valve in an emergency. To prevent tampering, the valve must be at least 2.3 m (7 ft) above grade. The discharge requirements for emergency discharge systems are found in CSA B52 Part 7.3. They are the same as for pressure-relief devices. The point of discharge must not be less than: A E 4.6 m (15 ft) above the adjoining ground level or an accessible roof level 7.6 m (25 ft) from any window, ventilation opening, or exit. Figure 23 – Emergency Discharge Line Diffuser Roof Wall Dual Relief Valves Receiver Glass Fronted Box 4-23 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 24 – Emergency Discharge Valve and Equipment Shutdown Switch 4-24 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Objective 2 Describe refrigeration system leak test procedures. System Leak Testing After a refrigeration system is installed, repaired, or modified, the entire system must be thoroughly inspected for leaks. Every refrigerant-containing part of a system that is erected on the premises, except factory tested components, must be tested and proved tight after installation and before operation. Positive Pressure Pneumatic Testing Inert gases, such as dry nitrogen or carbon dioxide, are used as the testing medium. Gas is supplied from high-pressure cylinders, connected through a pressure-reducing valve to either the high- or low-pressure side of the system. CAUTION Oxygen and flammable gases must not be used for system pressure testing. An explosion could result. Nitrogen gas is commonly used for refrigeration system leak testing. Nitrogen is an asphyxiant. A large leak of nitrogen test gas can also be hazardous. Ensure the surroundings are well ventilated when leak testing a system. Personal gas monitors should be worn. The following field pressure test procedure is based on the requirements of IIAR Standard 5: Start-up and Commissioning of Closed Circuit Ammonia Refrigeration Systems, ASME B31.5, and CSA B52 Mechanical Refrigeration Code. CAUTION Unlike hydrostatic testing, pneumatic testing can result in devastating explosion. Take measures to protect personnel from the potential of piping component rupture during pneumatic testing of systems. Preparation for Pressure Testing All piping joints must remain un-insulated. Welded joints should remain unpainted and free of rust, dirt, oil, and other foreign materials, until after field leak testing is complete. The field test should be witnessed by the jurisdictional inspector. 4-25 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Before the test, the following preparations must be made: Examine all piping before pressure is applied, to ensure that it is tightly connected. Any refrigeration component that has been factory tested and may be harmed by the test pressure must be valved off and isolated. These include pressure switches and pressure transducers. All safety pressure relief devices that may be subject to test pressure must be removed. Cap or plug the openings. Manually open all solenoid, pressure-regulating, check or other control devices using their manual lifting stems. Cap, plug, or lock shut all valves and devices leading to the atmosphere. Open all other valves. Essentially, the valve lineup must permit the high and low-pressure sides of the system to be initially tested together, at the same pressure. Pressurization Procedure The test gas shall be introduced into the system gradually through the charging valve, or other suitable injection point installed with a stop valve. The test pressure shall be verified using a calibrated pressure gauge located on the part of the system being tested. No leak repairs shall be made while that part of the system is under pressure. A suitable dry gas such as nitrogen or air must be used for field leak testing. The following fluids must not be used for leak testing an ammonia refrigeration system: Oxygen or any combustible gas or combustible mixture of gases Carbon dioxide Halocarbon refrigerants Water or water solutions. The high side and low side of the system must be leak tested at the greater of the design pressure shown in CSA-B52 Table 4, or the system design pressure. This takes into account that systems A E may be designed for higher pressure than the minimum stipulated in Table 4. The system must be held under pressure until proven tight with no more than a 1% loss in pressure, after accounting for temperature changes. Dry nitrogen gas is supplied in large, pressurized cylinders. For safety, and to prevent over-pressurization of the system, the pressure testing cylinder must have the following: A shutoff valve. A bleed valve. A pressure regulator located between the nitrogen cylinder and the refrigeration system, to control the supply pressure. An adjustable pressure relief valve located on the refrigeration system side of the regulator. The valve must be rated for the full discharge capacity of the nitrogen regulator, and set to the relevant test pressure. Calibrated cylinder and line pressure gauges. 4-26 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 A pressure test is performed as follows: 1. Connect the nitrogen cylinder to the system charging valve. Set the pressure regulator for the required low-side minimum design pressure, according to CSA-B52 Code, Table 4. For example, R-717 has a low side minimum design pressure of 951 kPa. R-134a has a A E low side minimum design pressure of 593 kPa. If the design pressure is higher than that specified in Table 4, set the regulator to the design pressure. 2. Gradually raise the pressure in the system. A preliminary test at up to 170 kPa may be applied to find major leaks. For large systems, the pressure should be gradually increased to one-half of the test pressure. Then, increase the pressure in steps of approximately one-tenth of the test pressure until the required test pressure is reached. When the line pressure gauge reads the required low side pressure, shut off the cylinder. 3. Isolate the high and low sides from each other, so that the high side can be tested at high side pressure without damaging the low side components. To do this, close the hand expansion valve or solenoid valve at the evaporator inlet. Close the compressor discharge valve. 4. Set the pressure regulator for the minimum high side design pressure, according to CSA B52 Table 4. The minimum design pressure varies whether the condenser is air cooled, or water cooled. For example, R-717 has a minimum high side design pressure A E of 2016 if air cooled, and a minimum design pressure of 1473 kPa if cooled with water or with an evaporative condenser. If the design pressure is higher than that specified in Table 4, set the regulator to the design pressure. 5. Open the nitrogen cylinder shutoff valve, and increase the high side pressure. 6. Close the cylinder shutoff valve, and disconnect the nitrogen cylinder. CAUTION A pneumatic pressure applied to a refrigeration piping system under test shall not exceed 130% of the design pressure of any system component. Leak Testing The system can now be leak tested at low side pressure. Because the test gas (nitrogen) is inert, it is hard to detect by any method other than a soap bubble test. Examine all joints, regardless of connection method. Some leaks may be difficult to find using a bubble test. As an additional requirement, CSA B52 states that the system must sustain the test pressure for a minimum of 2 hours. If leaks are found, they must be repaired. Weld joints must have the defective weldment removed, and the joint must be re-welded. Brazed joints can be cleaned, re-fluxed, and re-brazed. CAUTION Piping systems and components must be depressurized before repairs are made. Never attempt to make repairs to a pressurized piping system. After repairs are made, the system is leak tested again until proven leak free. With HFC and HCFC refrigerants, a small amount of “tracer” gas may be added to the system to assist with leak detection. The tracer gas is the same refrigerant normally charged in the system. With the tracer gas added, sensitive electronic refrigerant leak detectors can be used. Tracer gas is not used with ammonia systems. 4-27 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration If no further leaks are discovered during the pressure test, the system may be left pressurized for about 24 hours. The nitrogen or carbon dioxide cylinder must be disconnected if the system is left unattended. Disconnection will prevent accidental over-pressurizing of the system if any of the valves between the cylinder and the system should leak. If the pressure in the system has not changed after this period (allowing for pressure changes due to changes in ambient temperature), the gas is bled off from the high and low sides of the system. Any controls or relief devices previously removed for the pressure test are re-installed. The system is now ready for drying and charging. Sub-Atmospheric Pressure Testing Detecting leaks in a system that operates below atmospheric pressure is more difficult. Leakage of air into the system will cause the purge unit to cycle more often than usual, resulting in a loss of refrigerant because it is impossible to totally separate air from the refrigerant during purging. To test a sub-atmospheric pressure refrigeration system for leaks, it is necessary to shut down the compressor and break the vacuum by pressurizing the system with dry nitrogen. This is completed as follows: 1. Shut down the compressor and place the purge switch on manual. 2. Connect a nitrogen cylinder to the charging valve. 3. Open the charging valve fully. 4. Set the pressure-reducing valve on the test unit to the pressure recommended by the manufacturer and slowly open the shutoff valve on the cylinder. 5. Observe the pressures on the evaporator and condenser gauges. Close the nitrogen cylinder shutoff valve when both gauges read an adequate positive pressure. Caution should be taken not to over pressurize the system; otherwise, the rupture disc in the chiller will be damaged. 6. Test all the joints using an electronic refrigerant leak detector or a soap and water solution. 7. Make repairs to any leaks found. Start the purge system and allow the necessary time for the non-condensable gases to be released. Leak Detectors Leak detectors are used to find leaks during construction of new plants and for on-going checks while systems are in operation. These include: Electronic Leak Detectors Litmus Paper Phenolphthalein Paper Sulfur Candles Soap and Water Two most common leak detection methods are the soap and water test and the electronic leak detector. 4-28 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Electronic Leak Detector The electronic leak detector (Figure 25) draws vapour through a tube fitted with a sniffer at the end. The sniffer is moved over the area where a leak is suspected. It measures the electrical resistance of the vapour sample. As long as air is drawn into the detector, the resistance does not change. As soon as the sample contains refrigerant, the change in resistance causes the detector to react. The presence of refrigerant is displayed on a meter, accompanied by a light and a buzzer sound. Sniffers are available to detect ammonia, HFCs, and HCFCs. Figure 25 – Electronic Leak Detector Litmus Paper Detector Leaks in ammonia systems may be detected by using strips of wetted litmus paper. Litmus papers are formulated to change colour in the presence of an acid or a base. Blue litmus paper turns red when exposed to acidic conditions and red litmus paper turns blue when exposed to basic conditions. Since ammonia dissolves in water to produce a basic ammonium hydroxide solution, red litmus paper turns blue in the presence of ammonia. When moved about a joint or valve spindle, a change in the paper colour to blue indicates an ammonia leak. Phenolphthalein Paper Often confused with litmus paper, phenolphthalein paper is used when a more sensitive test for ammonia is required. These papers are white in colour and change to red when exposed to a solution with a pH greater than 8.3, which includes ammonium hydroxide solutions. Sulfur Candle Test Ammonia leaks can also be detected using a sulfur candle. When a flame from the sulfur candle comes in contact with leaking ammonia, a thick white smoke is created. CAUTION The sulfur dioxide formed when a sulfur candle is burned is an irritating and toxic gas. This method should only be used in well ventilated locations. 4-29 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Soap and Water Leaks in any pressurized system can be found by using a solution of soap and water. Simply brush the solution on the area where the leak is suspected. If a leak is present, bubbles will appear. If this method is used, the solution should be washed off after the test, or it will dry and the soap will collect dirt on the outside of the system. 4-30 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Objective 3 Describe how a refrigeration system is dried and charged prior to start-up. System Drying and Evacuating Any moisture or water vapour present in a refrigerating system causes serious operating problems. When exposed to low temperatures produced by the system, moisture entrained in the refrigerant causes icing or “freeze-up” at the expansion valve. In systems using HFCs and HCFCs as the refrigerant, acid is formed that reacts with oil to produce a sludge which corrodes metal parts. In addition, the acids will remove copper from heat exchanger surfaces and redeposit it at points of high temperature, such as bearings and compressor exhaust valves, a process called “copper plating”. Quite often during operation, large, specially designed, temporary driers are installed in the system. As the liquid refrigerant passes through the drying agent, any existing moisture is absorbed. Before charging an empty refrigerating system, the entire system is put under a very high vacuum (5000 microns or 0.67 kPa absolute pressure) with a special vacuum pump. The system compressor must not be used since it is not designed for this purpose and could be seriously damaged. Evacuation of the system should not be attempted unless the temperature of the surrounding air is 20°C or higher. As the air is removed from inside the system, the reduction in pressure will cause the moisture to evaporate and be removed with the air. Some switches and controls may not have vacuum protection. Before evacuating the system, these switches should be valved off or disconnected. After sufficient vacuum has been obtained, the vacuum is broken by admitting dry nitrogen gas. The gas is then evacuated from the system by the vacuum pump to once again produce a high vacuum. This second vacuum will remove the last traces of moisture from the system. Allow the system to remain under vacuum as recommended by the refrigeration system manufacturer. If the system pressure does not increase, the system is free of leaks and moisture. The system is now ready to be charged with refrigerant. Air remains inside a system if it is not adequately evacuated before charged with refrigerant. Air is a non-condensable gas that accumulates in the condenser during operation, causing high compressor discharge pressures and temperatures. System Charging Before proceeding with the actual charging process, the entire system must be checked to ensure all the components are ready for operation. Valves are opened where necessary, controls are adjusted to the required setting, and the sequence of controls and interlocks is tested. CAUTION Only technically-qualified persons, trained and certified in the handling of refrigerant and operation of refrigeration systems, should attempt to charge refrigeration systems. The information in this objective is general in nature, and designed to help Power Engineers understand the refrigeration system charging process. 4-31 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration The initial refrigerant charge is added to the high-pressure side of the system. The refrigerant drum is connected to the liquid charging valve located in the liquid line between the liquid shutoff valve on the receiver or liquid receiver and the expansion valve. A dehydrator should be installed in the line between the refrigerant drum and the liquid charging valve. Sometimes a pressure gauge is also connected in the charging line, although it is not absolutely necessary since the pressures in the system can also be observed on the compressor panel gauges (see Figure 26). CAUTION When handling refrigerants, goggles must be used for eye protection since even safe refrigerants can cause serious injury by freezing the moisture in the eyes. Neoprene gloves and protective clothing must be worn to prevent freeze burns. Protective breathing equipment may be worn or kept close at hand. Charging ammonia refrigeration systems must involve several people, one of whom should be an observer with appropriate PPE and the means to summon help. All unnecessary personnel should be clear of the area when ammonia systems are charged. The air in the charging line should be purged by leaving the connection at the charging valve slightly loose and cracking open the drum valve. After the air has been forced out of the charging line, the drum valve is closed and the connection to the charging valve tightened. The drum is then inverted so only liquid will pass through the charging line. Figure 26 – Refrigeration System Charging Arrangement Liquid Charging Valve Liquid Line to Evaporator Pressure Sight Glass Gauge Suction Line Water Cooled Condenser Liquid Line Dehydrator Shutoff Valve Discharge Valve Suction Valve Refrigerant Drum (Courtesy of Trane) 4-32 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Figure 27 shows a simplified overview of refrigeration system valves. The “King” valve is also called the liquid line shutoff valve. The charging cylinder and charging arrangement are simplified for illustrating the charging procedure. Figure 27 – Charging Liquid to a Refrigeration System In preparation for charging the system in Figure 27: 1. Check the liquid line shutoff valve (King valve) is closed at the receiver outlet. 2. Open the compressor suction and discharge valves. 3. Turn on the condenser cooling water supply or start the condenser fan if an air-cooled or evaporative condenser is used. 4. Move the thermostat to its lowest setting if a thermostat controlled solenoid stop valve is used in the liquid line, or manually open the valve. The liquid charging valve is opened and the drum valve is then cracked open to admit liquid refrigerant slowly into the system. If the refrigeration system uses ammonia, the vacuum is broken with gas and not liquid. To charge with gas, the cylinder must be upright. Ammonia gas is fed until the pressure in the system reaches about 690 kPag. It will take a considerable amount of refrigerant to break the existing high vacuum in the system and raise the system pressure to atmospheric pressure. Once the pressure starts rising above atmospheric pressure and above the setting of the low-pressure cutoff switch, the compressor starts. The system now operates normally, except that the liquid flowing into the evaporator is supplied from the refrigerant drum. The evaporator must have a source of heat energy during the charging operation (just as in normal operation) in order to evaporate the refrigerant. Increase the system load as much as possible, by opening doors and operating evaporator coil fans. While charging, the refrigerant passes through the system, condenses, and accumulates in liquid form in the condenser or liquid receiver. During the charging procedure, the liquid line shutoff valve remains closed. Charging continues until the system contains the amount of refrigerant required by the manufacturer, which can be determined by placing the refrigerant drum on a weigh scale. If the receiver is equipped with a sight glass, this can also be checked. 4-33 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration When the correct amount of refrigerant has been added to the system, the drum valve and the charging valve are closed. The liquid shutoff valve (King valve) can now be opened. The flow of refrigerant should be monitored through the sight glass. If bubbles appear in the flow after the system has settled down to normal, additional refrigerant may be required. In small refrigeration systems, frost formation on the compressor suction line indicates overcharging. This causes high suction and discharge pressures, and high compressor power consumption. A secondary concern is that liquid refrigerant could be forced into the compressor, especially in capillary tube systems. The operation of the entire system should be carefully monitored to ensure it is functioning as required. If everything appears normal, the charging line is disconnected. Disconnecting should be done with care since the line contains some refrigerant under pressure. After the system has been in operation for some time, it may be necessary to periodically add a small amount of refrigerant. The refrigerant is often added in its vapour state by connecting the drum in upright position to the suction line of the compressor. Care must be taken so no liquid refrigerant is carried over from the drum to the compressor. 4-34 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Objective 4 List the steps for adding oil to an in-service refrigeration compressor. Refrigeration compressors are internally lubricated. Because of this, compressors pass oil through the discharge piping along with refrigerant vapour. One consequence is that compressor sumps gradually run out of lube oil. For this reason, oil must be returned to the crankcase from the system, or fresh oil must be added. Refrigeration compressors are sealed units to prevent the escape of refrigerant. One cannot merely pull out a dipstick, remove a cap, and add oil with a funnel. Hand pumps and a pressure rated hoses with quick-connectors are commonly used. Oil addition can be a dangerous proposition, especially when the refrigerant in the system is pressurized, toxic, and flammable (such as with an ammonia system). Adding oil involves accessing the compressor crankcase, which contains refrigerant vapour at low side pressure. Refrigerant leaks and oil sprays may occur during the procedure. Ensure the ventilation system is running at full capacity while adding oil. Personal protective equipment, specific site procedures, and training are necessary before attempting to add oil to a refrigeration compressor. Adding Oil to a Compressor Refrigeration compressors have bull’s eye gauge glasses to show crankcase oil level. Normal oil level is about ½ the glass. If the oil level is low, first try to return lube oil from the oil separators back to the crankcase. If the oil separators are equipped with float-operated drain valves, this happens automatically. Otherwise, oil must be returned manually. Figure 28 shows an arrangement for returning oil to the crankcase from a separator. Figure 28 – Oil Return System 4-35 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration If the oil level is still low after returning oil to the crankcase, oil must be added. Oil is added by charging it into the crankcase with a hand pump. This may be done with the compressor in operation, provided that: The crankcase pressure is not excessive (690 kPag maximum is generally specified by pump manufacturers). The compressor manufacturer permits it. Precautions must be taken to ensure air and moisture do not enter the system when adding oil. To add oil: 1. Obtain a container of the correct oil. 2. Place the suction of the oil pump into the oil container. 3. Prime the oil pump discharge line and bleed off any air through the oil pump, oil line, and the quick connector before attaching the hose to the compressor oil charging valve. 4. Open the charging valve and operate the hand pump until the oil level is half-way up in the gauge glass. Figure 29 shows a hand pump used for charging oil into a compressor. Figure 29 – Oil Charging Arrangement When oil charging is complete: 1. Close the fill valve. 2. Disconnect the hose from the fill valve. 3. Return the oil and pump to their storage area. 4. Clean up oil spills. Use of a hand pump is the easiest way to add oil. However, if a hand pump is unavailable, the compressor suction can be used to draw oil into the crankcase. This method must be acceptable to the compressor manufacturer, and must be an approved plant procedure. 4-36 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Refer to Figure 30: 1. While the compressor is in operation, the suction valve is throttled. The compressor is allowed to continue operating in order to create a slight vacuum in the crankcase. 2. One end of a hose is connected to the compressor oil fill valve. The other end is raised, and the hose is filled with oil to remove all air. The free end is then submerged in a container with fresh lube oil. On Track Care must be taken to ensure that the proper oil type is added to the system. Proper selection is dependent on refrigerant type and refrigerant operating temperature. 3. The oil fill valve is now cracked open, slowly allowing oil to be sucked into the crankcase until the proper level on the crankcase oil level indicator is reached. Then, the oil fill valve is closed. 4. The compressor suction valve is then opened fully and the system is returned to normal operation. On Track Never allow the end of the hose to come above the surface of the oil. This will draw air into the system. 5. Remove the hose and cap the oil fill valve fitting. Figure 30 – Adding Oil to a Refrigerating Compressor Crankcase Oil Fill Valve Hose Oil Container Each compressor manufacturer supplies detailed instructions concerning the correct procedure for adding oil. Regardless of the method used, extreme care must be taken not to introduce contaminants such as air, water, or dirt into the machine. Also, ensure the oil compatibility, quantity, and viscosity are correct. 4-37 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Draining Oil from Compressor and System CAUTION Do not drain oil from systems that are not isolated from system pressure! Compressor Oil When excess oil must be removed from the compressor crankcase, or the oil has to be changed 1. Pump down the compressor until the pressure in the crankcase approximately equals atmospheric pressure. This is done by slowly closing the compressor suction valve and monitoring the compressor suction (or crankcase) pressure gauge. 2. Shut off the compressor. 3. Isolate the compressor suction and discharge valves. 4. Lock out the compressor so it cannot be started until the job is finished. Follow site specific lockout procedures. 5. Drain the oil by either opening the drain valve or removing the drain plug. Exercise caution when removing drain plugs as crankcase pressure will blow out oil and refrigerant vapour. Oil Pots Oil separators that do not automatically drain to the crankcase should be manually drained at regular intervals. In many plants, oil separators (and other system components) do not return oil directly to the compressor. Rather, they may drain to oil pots. These are small pressure vessels that are left open to the system to gather oil. They are equipped with valves so that they can be isolated from the system and safely emptied of oil. Some are piped to return oil to the compressor. An oil pot installation is shown in Figure 31. Valve A is a quarter-turn, spring-loaded “dead man” valve, which closes automatically. Valve B is a guard valve used to isolate the oil return line. Valve C lets oil flow from the separator into the oil pot. Other valves (not shown) allow oil to flow into the pot from other system low points. Valve D permits any liquid refrigerant that accumulates in the oil pot to evaporate and re-enter the suction line. A safety valve must be installed on the oil pot in case all the valves are closed. Such a situation could over-pressurize the oil pot. 4-38 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Figure 31 – Oil Pot Installation In normal operation, valve C is open. When the separator is full of oil and ammonia, a float-operated trap (not shown) dumps oil and ammonia into the oil pot. Valve D is also normally open. The pot is thus maintained at low-side pressure. Any refrigerant liquid that enters the pot evaporates and is entrained with the compressor suction flow. Valves A and B are normally closed, and the end of the drain pipe is capped. Liquid ammonia is less dense than lube oil. Boiling ammonia in the oil pot creates at frost line at the interface between the liquid ammonia and the oil. As the oil pot fills, the frost line moves higher up the side of the oil pot. When the oil pot is full, it needs to be drained. The following is a typical procedure. Refer to Figure 31. 1. Close the liquid supply valve (C) and wait. Sufficient time must be given to allow ambient heat to fully evaporate residual liquid ammonia from the oil pot. This is indicated when all the frost is melted from the exterior of the oil pot. Depending on the location and service of the oil pot, this could take up to a full day. 2. Gather PPE, an oil receptacle, and work permit. Arrange an operator to be present as a dedicated backup. The backup (“buddy”) must have full PPE and a method of summoning help. PPE may include chemical resistant gloves, aprons, safety glasses with face shield, and full-face respirator. Some plants may require eyewashes and safety showers to be tested before draining the oil pot. 3. Close the vent line valve (D). This fully isolates the oil pot from the system. 4. Place the drain receptacle to collect oil, and carefully remove the plug or cap from oil drain line. The plug is intended to prevent seepage of oil while the pot is in service. There is often residual oil from the previous drain; therefore, proceed with caution when removing the plug. 5. Open the “dead man” valve (A). Oil should not flow. Never prop open the dead man valve. 4-39 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration 6. While holding the dead man valve open, slowly open the oil drain shut-off valve (B) to start the flow of oil from the pot. The oil drain valve is used to throttle the oil flow from the pot to the receptacle. If there is an unexpected increase in oil flow, the dead man valve can be released, quickly stopping the oil flow. The oil flowing during the draining process often appears brown in colour and frothy. The oil drain process is complete when the oil flow begins to be intermittent with vapour from the pot. 7. Securely close the oil drain valve (B) while holding the dead man valve (A) in the open position. This is to enable oil or vapour contained in the line between valve A and B to flow out. Then, allow the dead man valve to close. 8. Open vent valve (D) and supply valve (C). 9. Crack open the dead man valve (A) to ensure that isolation valve (B) is holding tight. If not, tighten valve B. Then, crack the dead man valve again to ensure valve B is holding. 10. If the isolation valve (B) is holding, reinstall the pipe plug or cap at the outlet of valve A. 11. Carefully place the oil receptacle in a well-ventilated area to let any ammonia vapour absorbed in the oil to off-gas. 12. After the oil has settled, record the amount of oil drained. This will provide information on compressor oil consumption. 13. Ensure the oil is collected and disposed according to jurisdictional and environmental regulations. 4-40 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 Objective 5 Describe the start-up and shut-down procedure for a compression refrigeration system. Refrigeration system startup and shutdown procedures are supplied by the equipment manufacturers. The following material provided is generic. Operators must review the supplied manufacturer’s recommendations and plant standard operating procedures prior to starting or stopping the plant. Reciprocating or Rotary Compressor System Start-Up The following general guidelines apply to systems equipped with reciprocating and rotary compressors. These guidelines apply to new systems and systems which have been out of operation for a prolonged period of time for maintenance or seasonal shutdowns. Before Starting the Compressor: 1. The operator should become familiar with the entire refrigeration system and all its accessories before operating the equipment. 2. Check that power is available to circuit breakers, compressors, water pumps, and cooling tower. 3. Ensure the high- and low-pressure shutdown switches have been properly set up by a qualified operator or instrumentation technician. 4. Ensure all other instrumentation work on the refrigeration system has been completed. 5. Ensure all mechanical work on the compressor and the rest of the system has been completed. 6. If the compressor is equipped with an oil sump heater, ensure it is energized and the oil temperature is high enough to drive off any refrigerant, prior to start up 7. Check the operation of system interlocks. For example, the compressor should not be able to start if the fans in an air conditioning system are not operating, or the evaporative condenser is not running properly. 8. Open block valves in the cooling water supply and return lines of water cooled condensers, or start fan motors of air cooled or evaporative condensers if not tied in with the compressor starting system. Open the water supply valve to the evaporative condenser sump and check the water level. 9. Check oil level in the compressor: it should be at or above the centre of the sight glass. 10. Ensure the lubricators (if equipped) are full of oil. 11. All block valves in the system should be open, except bypass valves used for other purposes. 12. Solenoid valves in the various liquid lines should be closed and on automatic control. 13. 1Suction, discharge and oil pressure gauges should be connected, and any valves in the connecting lines should be open. 14. Be sure all work permits have been completed, signed off and returned to the maintenance coordinator or the control room. 4-41 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Starting the Compressor When all the above conditions are satisfied, the compressor can be started using the following procedure: 1. On smaller capacity compressors, open the suction and discharge valves. 2. On larger compressors open the suction and discharge valves, as well as the bypass valve between the suction and discharge. The reason for opening the bypass valve is so the compressor can be started in an unloaded state, which avoids an excessive starting torque resulting in high power draw. Once the compressor is up to speed, slowly close the bypass valve. If there is no bypass valve, close the suction valve and open the discharge valve before starting. When the compressor is up to speed, slowly open the suction valve. 3. Check the whole system, observing temperature and pressure gauges. Any operating difficulties should be immediately corrected before proceeding. 4. Check controls for proper operation and reset if necessary. 5. Check superheat setting of thermostatic expansion valves and adjust if required. 6. Check the operation of the water regulating valve in the water supply line to the condenser. If the compressor discharge head pressure is too high, adjust for increased condenser water flow. 7. Check the liquid refrigerant sight glass for bubbles. If any appear, refrigerant may need to be added, but not before other checks are made. 8. Check the oil level in the crankcase after the compressor has run for about 15 to 20 minutes. If the compressor is pressure lubricated, check the oil pressure. 9. Check the entire system with a leak detector. Reciprocating or Rotary Compressor Refrigeration System Shutdown When a system has to be shut down for a prolonged period of time, it should be pumped down and all refrigerant should be stored in the liquid receiver. This practice will prevent unnecessary strain on the low-pressure side of the equipment and loss of refrigerant while the system is shut down. For a direct expansion evaporator, it is essential that the evaporator be pumped down every time the compressor is shut down to make sure all refrigerant is removed from the evaporator. This prevents liquid slugging and compressor damage on start-up. The following procedure must be followed for a system that has a direct expansion evaporator: 1. Close the liquid line shutoff or “king” valve on the receiver outlet to stop the flow of refrigerant to the evaporator. 2. If a solenoid valve is used in the liquid line, it should be held open so that all liquid can be withdrawn from the line. 3. With the entire system in operation, lower the pressure on the low side until compressor suction gauge indicates 14 kPa. It may be required to manually hold the low pressure cut- off in the closed position. 4. As soon as pressure reaches about 14 kPa (2 psi), stop the compressor and close the compressor suction and discharge service valves. Never pump down below 7 kPa to 14 kPa (1 to 2 psi), since a slight positive pressure is needed to prevent air from being drawn in through minor leaks or the compressor shaft seal. Close all other valves in the system. The part of the system containing the refrigerant charge should be thoroughly checked for leaks. 4-42 4th Class Edition 3 Part B  Refrigeration Systems Operation and Maintenance Chapter 4 5. Close cooling water supply to the compressor and water-cooled condenser, if so equipped. If equipment is subject to freezing temperatures, all water should be drained. 6. If the system is equipped with an evaporative condenser, close the makeup water supply, drain the water, and flush the condenser. 7. Open the master power switch for the system and lock it in the open position. Ensure that proper tags indicate the lockout and the reason for the shutdown. Centrifugal Compressor Chiller System Startup In older systems, most of the auxiliary equipment, such as the chilled water circulating pump, condenser cooling water pump and cooling tower fan, had to be started individually. In modern systems, however, all equipment is electrically interlocked and started in the proper sequence simply by depressing the start button on the control panel. Larger complex chiller systems may still need some auxiliary equipment started manually. Prior to starting up the centrifugal compressor of a chilled water system after a prolonged shutdown, the following “general” procedure should be followed: 1. Check oil levels in the compressor, pumps, motors, and gear boxes. An abnormally high oil level in the compressor oil sump indicates refrigerant absorption by the oil. The refrigerant can be driven out of the oil by energizing a special oil sump heater installed for this purpose, or by raising the thermostat setting if the heater is in operation already. Ensure that the heater is in operation. 2. Check the refrigerant level. 3. Open the stop valves in the chilled water system. Check the chilled water expansion tank for proper level. 4. Check the water level in the cooling tower. Open the makeup water valve and all the valves in the cooling water system. 5. Close the main circuit breakers. 6. Energize the power to the electrical control system. If the system is equipped with pneumatic controls, make sure the required air supply pressure is available. 7. Start the purge unit to remove any air that may have entered the system. The unit should operate for at least 10 minutes before the compressor is started (time depends on the size of the system). 8. If the compressor is equipped with a separate oil pump, turn the pump on. Operate it for 10 minutes before starting the compressor. The oil temperature should then be up to the required minimum. When the pre-startup procedure has been properly followed, the system is ready to be started up. 1. Set the chiller demand limiter to its lowest setting. 2. Start the compressor by initiating the start sequence through either the Building Management Control System (facility control system) or a local panel. This starts the chilled water circulating pump. The chilled water flow will be proven and the return water temperature control will be enabled. 3. If the return chilled water temperature is at or above the cut-in setpoint, the control relay will start the condenser cooling water pump. 4. The cooling tower fan may operate, depending on whether the condensing water temperature at the time is above or below control point temperature. 5. At this point, if all protective controls have been energized and have been satisfied, the compressor motor will start. 4-43 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration During the starting sequence, the inlet damper or vanes are automatically held in the closed position. This allows the compressor to start in an unloaded state, thus reducing starting torque and starting current draw. After the compressor has reached normal speed, control of the damper or vane operator is taken over by the thermostat sensing the temperature of the water leaving the chiller. After the compressor comes up to normal speed and takes on load, check oil and refrigerant levels continually for the next 30 minutes. Turn on the water supply to the oil cooler and adjust